Thermoplastic polyester resins such as polyethylene terephthalate are excellent in heat resistance, chemical resistance, weather resistance, mechanical characteristics and electric characteristics, among others, and, therefore, they are used in various fields of industrial application, for example as injection molding materials, fibers and films. However, still higher levels of mechanical characteristics and heat resistance are required of them. To meet those requirements, attempts have been made to improve such characteristics generally by incorporating various inorganic particles. However, this measure may cause problems: the surface appearance of products may be impaired, or a fibrous inorganic material incorporated may be oriented in the step of injection molding to cause anisotropy.
The above drawbacks of inorganic particles are generally thought to arise from insufficient dispersibility of inorganic particles and/or an excessive size of dispersed particles. Thus, a technology of finely dispersing inorganic particles has been desired.
As regards layered materials, in particular layered phyllosilicates, among inorganic particles, a technology of facilitating the exfoliation of layered phyllosilicates along each foliated platelet to make it easy to finely disperse them by intercalating a high-molecular compound (intercalant polymer) such as polyvinylpyrrolidone between layers to give intercalation compounds has been disclosed (Japanese Kokai Publication Hei-09-118518). The above-cited publication discloses such intercalation compounds but does not disclose any technology of exfoliating the intercalation compounds and finely dispersing them in thermoplastic polyester resins. In fact, it has been difficult to finely disperse layered phyllosilicates in thermoplastic polyester resins.
Separately, the present inventors previously found that for causing exfoliation of layers of a layered phyllosilicate to thereby finely disperse the same in a thermoplastic polyester resin, it is particularly effective to treat the layered phyllosilicate with a polyether compound to convert the same to a clay-derived intercalation compound and disclosed a technology relative to a polyester resin composition comprising a thermoplastic polyester resin and a clay-derived intercalation compound (Japanese Kokai Publication Hei-10-259016, Japanese Kokai Publication Hei-10-310420). That technology has made it possible to increase the mechanical characteristics, such as elastic modulus, and heat resistance without causing any impairment in surface appearance or warpage. However, in a molten state, the thermoplastic polyester resin reacts with the polyether compound and the resin is decomposed thereby and, therefore, the thermal stability in molten state cannot be said to be high enough and the strength of moldings tends to decrease due to residence in molten state. In certain instances, as the resin repeatedly experiences a molten state, the viscosity of the resin decreases and the processability into moldings becomes degraded.
On the other hand, a technology is disclosed which uses a polyfunctional compound having two or more epoxy or carbodiimide groups in each molecule to increase the molecular weight of a polyester resin reduced in molecular weight as a result of decomposition (e.g. Japanese Kokai Publication Sho-46-5389, Japanese Kokoku Publication Sho-47-13860, etc.). It is generally known that the above technology is effective in retaining the impact strength, in particular the strength after exposure of high-temperature, high-humidity conditions, of thermoplastic polyester resins. However, there remain the problems of degradation in processability into moldings; for example, the fluidity of the resin decreases upon experience with a molten state.
Further, the present inventors previously disclosed an invention directed to a method of production which comprises melt-kneading a mixture of diol-treated inorganic particles and a thermoplastic polyester resin and then performing a polymerization procedure (Japanese Kokai Publication Hei-10-306205) and an invention concerning a production method which comprises melt-kneading a mixture of a bishydroxyalkyl terephthalate and inorganic particles together with a thermoplastic polyester resin to thereby cause polymerization (Japanese Kokai Publication Hei-11-140286). However, both methods may require a prolonged production time in certain cases.
Meanwhile, thermoplastic polyester polymer resins are widely used in automotive parts, electronic and electrical appliance parts, OA (office automation) equipment parts and the like. In these fields of application, materials are demanded which have not only good mechanical characteristics, heat resistance and moldability but also high flame retardancy. A number of methods have been proposed for rendering polyester resins flame-retardant using phosphorus compounds (e.g. Japanese Kokoku Publication Sho-51-19858, Japanese Kokoku Publication Sho-51-39271, Japanese Kokai Publication Sho-52-102255). However, the use of a phosphorus compound results in reduced mechanical characteristics, among others, and, in addition, raises the problem that if it is attempted to attain flame retardancy with a phosphorus compound alone, the resin melting on exposure to flames drips together with kindling seeds.
For suppressing the dripping of the resin exposed to flames, a technology is known which generally uses a fluororesin combinedly. However, although this prior art technology is effective to a certain extent in suppressing the dripping on the occasion of combustion, the use of a fluororesin in an amount of less than about 1 part by weight per 100 parts by weight of a polyester resin results in failure to attain stable flame retardancy due to poor dispersibility of the fluororesin. Conversely, the use thereof in an increased amount produces problems, such as an increased cost, an impairment in surface appearance of resin moldings due to poor dispersibility of the fluororesin, and decreases in fluidity and processability in molding.
Another technology has also been disclosed (Japanese Kokai Publication Hei-10-60160) which uses a layered phyllosilicate with a mean particle size of 0.5 to 300 μm in combination with a phosphorus-containing flame retardant. However, this technology is not yet fully satisfactory in dripping-suppressing effect since even when such a layered phyllosilicate with a mean particle size of 0.5 to 300 μm is used, the particle size is still too large. While the term “mean particle size” is interpreted to mean that the particles are indefinite in shape or are generally spherical, it is desirable for such particles to have a thin plate-like shape with a high aspect ratio so that a sufficient dripping-suppressing effect can be efficiently achieved.